Air displacement pipetters with disposable tips have been used in the medical industry for many years. The main reason for such continual acceptance comes from the fact that after each use the tip has traditionally been disposed of, thereby limiting the possibility of cross contamination between samples. However, because of the need to perform many tests from a limited account of sample quantity, polymerase chain reaction (PCR) was developed. PCR (covered under U.S. patents issued to CETUS Corporation), provides a method with which to produce many copies of a specific nucleotide sequence from a minute quantity of DNA.
Unfortunately, false signals can be generated following PCR amplification due to cross-contamination accuracy during carry-over between existing tips and air displacement pipetters. As the scrutinizing of these and many more tests have become more demanding, the need to eliminate any and all cross contamination is imperative. Even the smallest amounts of particles left behind on the barrel of the pipetter from previous tests can invalidate, or skew the evaluations of new test samples, causing hours or even days of laboratory research to be wasted. These errors could be contributed to operator use (which often causes splashing of the sample) or the sample could aerosol during aspiration of the fluid sample, or the fluids contaminated gases can flow into the tip upward into the calibrated barrel in the form of air borne contaminates.
The filtered disposable pipette tip was developed to help prevent such problems. However as shown below, the advantages of the existing designs do not meet the more stringent requirements for test evaluations of today's medical requirements.
Existing Filter Tips
U.S. Pat. No. 4,999,164 by Puchinger, U.S. Pat. No. 5,156,811 by White, U.S. Pat. No. 5,364,595 by Michael Smith and U.S. Pat. No. 5,496,526 by Gazit all disclose a filter tip which contains a plug of porus material frictionally engaged within the liquid chamber of a standard pipette tip. Referring to FIG. 1, a simplified view of pipette tip Tp is illustrated. Pipette tip Tp defines frustum shaped interior cavity 14 having apex end 16 opening to receive fluid F to be pipetted and truncated base 18 is exposed to receive pipetter barrel P (only partially shown) in a wedge type fit.
Plug filter N is utilized. Typically, plug filter N is wedged into pipette tip Tp. This is done on an individual basis.
By taking existing disposable pipette tips and installing a porous filter medium into the tip itself, the existing product now provides a barrier between the incoming sample and the calibrated barrel of the pipette (as shown in FIG. 1). This barrier is constructed to block passage of aerosols while permitting gas to flow through its pores. If the liquid contacts the plug, as noted by White, the particles of the plug will expand and completely block gas or liquid flow. This does help to prevent the possibility of some cross contamination between samples when disposable tips are exchanged.
Disadvantages
1. Existing filter media used in the previously mentioned applications are porous plastics (such as made by Porex) with a preferable pore size in the order of 25-40 microns, yet many tests require particulate retention below 0.6 microns to be effective.
2. The actual filter mass necessary to maintain the interference of the fit also reduces the amount of calibrated fluid volume designed to be held by the fluid tip. Not only does this not maximize the efficiency of the tip, but it poses the potential for an error to occur when the user must account for this problem by selecting a larger tip. This new tip may or may not fit the tube or receiving container the user wishes to access, (i.e., a 250 μl tip may only hold a volume of 150 μl after the filter medium is installed as shown in FIG. 1).3. As noted by White, when fluid does contact the plug the filter tip, its contents must be discarded. However, in cases where there is minimal sample material or no other sample available, this feature is disastrous because of the need to recover the sample locked within the tip.4. The filter mass also reduces the air flow of the pipetter and thus effects the calibrated volume of fluid which is drawn into the tip. Back pressure and flow rates are essentially proportional to material thickness. Thus, assuming the same pore size, the pressure drop would double when the material thickness doubles, (i.e., if 0.060 thick filter had a 8 psi pressure drop, then a 0.120 thick filter would be 16 psi and a 0.240 thick filter would be 32 psi). Average existing filter media range from 0.12 to 0.25 inch thickness.5. Cost is a major factor in any disposable, but $100-$125 per thousand is very high due to the fact the existing filters must be manufactured and installed individually.6. Existing pipetters are designed and calibrated without the use of this filter barrier within the tips. To maintain the original accuracy and precision specification of the pipetter, a minimum pressure drop is required. Thus, these prior art filter tips sometimes require the re-calibration of the pipetter to accommodate this filter mass.
It will also be understood that pipette tips having membrane filters are known. See Edelmann U.S. Pat. No. 4,267,729 and the prior art illustration shown in FIG. 1A.
Referring to FIG. 1A, pipette tip Tp 1 is illustrated defining frustum shaped interior cavity 14 with apex end 16 showing drawing fluid F and truncated case 18 attached to pipetter P (partially shown). Annulus A includes flat surface 20 extending normally to axis of symmetry 22. In Edelmann U.S. Pat. No. 4,267,729, conventional fastening, as by the use of adhesives, fastens membrane filter M to annulus A.
In Edelmann U.S. Pat. No. 4,267,729, membrane filter M has a special purpose. It is used as a stop for pipetted material. Specifically, in Edelmann, the volume delimited between membrane filter M and apex end 16 amounts the measured amount of fluid to be pipetted.
U.S. Pat. No. 4,461,328 by Kenny also discloses a pipetting device that comprises one or more pipette tubes to which a hydrophobic filter paper is secured. It also restricts the rise of an aqueous liquid in each tube, by the use of its hydrophobic paper similar to Edelmann, by allowing the passage of air from a liquid mass.
Disadvantages
1. In the above mentioned configurations, the devices are quite susceptible to plugging by particulate masses in the fluid samples. One example would be fibrin contained in blood samples which is a fibrous blood protein used in the clotting process.
2. Contamination is again a major concern when the liquid of any sample comes in contact with the filter medium or possibly the adhesives used for the attachment of the filter.
3. Fluid contacting the filter medium will create a meniscus, attaching itself to the medium, depending on the fluid's viscosity and surface tension of the medium during its dispensing cycle. This will give rise to inaccuracies in the precision and accuracy of the volumes dispensed.4. As with the porous plastic filter described by U.S. Pat. No. 5,156,811, the filter media of the above patents are primarily hydrophobic in nature and are only concerned with preventing fluids to pass the filter barrier while allowing gases to flow through. These gases, however, can contain air borne contaminants which freely enter the pipetter barrel through normal operation of the suction and dispensing cycle of the pipetter. Upon exchanging a new filter tip the contaminants may again flow from the contaminated barrel though the filter media and into the dispersed fluid sample.5. Another major objection is the difficulties and relative high cost associated with the manufacturing of these products. Liquid pipetting requires both high precision and accuracy of liquids dispersed within plus or minus 0.5% for some tests. Devices such as these have proven to be quite difficult to fabricate because of the close tolerances and multi-cavity tooling required in the medical disposable marketplace.